Current reduction in a single stage cyclic analog to digital converter with variable resolution

ABSTRACT

A converter ( 200 ) adapted to convert an analog input signal into a digital output signal includes an analog input terminal ( 205 ) for receiving the analog input signal, a Redundant Signed Digit (RSD) stage ( 210 ) coupled to the analog input terminal, and a digital section ( 220 ). The RSD stage is configured to receive the analog input signal at the analog input terminal, produce a first number of bits at a digital output from the analog input signal during a first half of a first clock cycle, provide a residual feedback signal of the analog input signal at the analog input terminal during a second half of the first clock cycle, and produce a second number of bits at the digital output from the residual feedback signal during a first half of a second clock cycle, the second number of bits less than the first number of bits.

TECHNICAL FIELD

This disclosure relates generally to Analog-to-Digital (A/D) converters,and more particularly to Redundant Signed Digit (RSD) A/D converterswith current reduction.

BACKGROUND

Advances in integrated circuit technology have enabled the developmentof complex “system-on-a-chip” ICs for a variety of applications such aswireless communications and digital cameras. Such applications areembodied in portable electronic devices for which low power and smallcircuit area are important design factors. Low power and low voltagecircuits are needed to decrease battery power requirements, which canallow for designs that use fewer or smaller batteries, which in turndecreases device size, weight, and operating temperature.

Such devices, however, receive analog input signals that are typicallyconverted to digital signals. Various conventional cyclic (algorithmic)A/D converters that achieve relatively lower power operation and with asufficiently high resolution in a relatively small area have beenachieved.

Even lower power while achieving the needed resolution is a continuingdesire. Thus any further reductions in power are desirable. Accordinglythere is a need for further reductions in power while achieving theneeded resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of some example embodiments will bebetter understood when read in conjunction with the appended drawings.It should be understood, however, that example embodiments are notlimited to the precise arrangements and instrumentalities shown. In thedrawings, like numerals are used to indicate like elements throughout.Furthermore, other desirable features and characteristics will becomeapparent from the subsequent detailed description and the appendedclaims, taken in conjunction with the accompanying drawings and theforegoing technical field and background.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring aspects of the illustrated embodiments.Additionally, elements in the drawings figures are not necessarily drawnto scale. For example, the dimensions of some of the elements or regionsin some of the figures may be exaggerated relative to other elements orregions of the same or other figures to help improve understanding ofthe example embodiments. In the drawings:

FIG. 1 is a schematic block diagram of a single-stage RSD A/D converterin accordance with example embodiments;

FIG. 2 is a schematic block diagram of an example embodiment of thesingle multi-bit/single-bit RSD stage of FIG. 1;

FIG. 3 is a schematic circuit diagram illustrating an example sub-ADCthat may be used to implement the multi-bit/single-bit RSD stage of FIG.2;

FIG. 4 is a schematic circuit diagram illustrating an example MDAC thatmay be used to implement the multi-bit/single-bit RSD stage of FIG. 2;

FIG. 5 is an example timing diagram illustrating control signals thatmay be used to perform an example 10-bit A/D conversion process with thesub-ADC of FIG. 3 and the MDAC of FIG. 4;

FIG. 6 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the first clockphase of the example 10-bit A/D conversion process;

FIG. 7 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the second clockphase of the example 10-bit A/D conversion process;

FIG. 8 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the third clockphase of the example 10-bit A/D conversion process;

FIG. 9 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the fourth clockphase of the example 10-bit A/D conversion process;

FIG. 10 is a simplified circuit diagram illustrating the configurationof the sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the fifth clockphase of the example 10-bit A/D conversion process;

FIG. 11 is a simplified circuit diagram illustrating the configurationof the sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the sixth clockphase of the example 10-bit A/D conversion process;

FIG. 12 is a simplified circuit diagram illustrating the configurationof the sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the seventh clockphase of the example 10-bit A/D conversion process;

FIG. 13 is a simplified circuit diagram illustrating the configurationof the sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the eighth clockphase of the example 10-bit A/D conversion process;

FIG. 14 is a simplified circuit diagram illustrating the configurationof the sub-ADC of FIG. 3 and the MDAC of FIG. 4 during the ninth clockphase of the example 10-bit A/D conversion process; and

FIG. 15 is a flowchart illustrating example processes included in amethod of A/D conversion according to an example embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The detailed description set forth below in connection with the appendeddrawings is intended as a description of some of the exampleembodiments, and is not intended to completely describe all possibleembodiments. That is, there is no intention to be bound by any expressedor implied theory presented in the preceding technical field,background, or the following detailed description of exampleembodiments. It is to be understood that the same or equivalentfunctions may be accomplished by different embodiments.

The terms “first,” “second,” “third,” “fourth” and the like in thedescription and the claims, if any, may be used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments described herein are, for example, capable of use insequences other than those illustrated or otherwise described herein.Furthermore, the terms “comprise,” “include,” “have” and any variationsthereof, are intended to cover non-exclusive inclusions, such that aprocess, method, article, or apparatus that comprises, includes, or hasa list of elements is not necessarily limited to those elements, but mayinclude other elements not expressly listed or inherent to such process,method, article, or apparatus.

FIG. 1 is a schematic block diagram of a single-stage RSD A/D converter.The A/D converter 200 includes a single multi-bit/single-bit RSD stage210 and a digital section 220. The digital section 220 has an alignmentand synchronization block 230 and a correction block 240. An analoginput signal (e.g., voltage) 205 is presented to an input of the singlemulti-bit/single-bit RSD stage 210 by way of a first switch 212. The RSDstage 210 provides a digital output signal to the digital section 220.The RSD stage 210 also generates a residual voltage signal (VR), whichis fed back to the input of the RSD stage by way of the first switch212. The first switch 212 is closed for the first cycle, in which theanalog input signal 205 is received, and then opened for the remainingnumber of cycles that it takes to complete converting the analog signalto a digital signal. Preferably, the feedback loop of the RSD stage 210is directly connected from the RSD stage output to the first switch 212,without any intervening circuitry such as a comparator. The number ofcycles to complete an A/D conversion of the analog input signal to adigital output signal depends on the number of bits in the digitaloutput signal. The digital bits output from the RSD stage 210 areprovided to the digital section 220, where they are aligned,synchronized, and combined to provide a standard format binary outputcode.

The architecture of FIG. 1 is capable of achieving significantreductions in total capacitance, area, and power. This is because,according to example embodiments, the single multi-bit/single-bit RSDstage 210 is initially configured to have a resolution of at least 2.5bits during a first clock phase of the A/D conversion, then reconfiguredto have a resolution of 1.5 bits during subsequent clock phases of theA/D conversion.

FIG. 2 is a schematic block diagram of an example embodiment 300 of thesingle multi-bit/single-bit RSD stage of FIG. 1. The RSD stage 300includes the input terminal 205 at which the analog input signal (VIN)is applied and a first switch 305 that is used to selectively apply theanalog input signal (VIN) to the node 307. The RSD stage 300 alsoincludes a feedback switch 315 that is used to selectively apply aresidual voltage feedback signal (VR) to the node 307.

The RSD stage 300 further includes a first, second, third, fourth,fifth, and sixth comparators 302, 304, 306, 308, 310, and 312,respectively. Because the RSD stage 300 has six comparators, it canachieve a maximum resolution of 2.5 bits. Although the six-comparatorconfiguration illustrated in FIG. 3 is preferred, alternativeembodiments may have more than six comparators. In other words,alternative embodiments may achieve resolutions that are greater than2.5 bits. Each of the comparators 302, 304, 306, 308, 310, and 312 has apositive input terminal that is connected to the node 307. Depending onthe state of the first switch 305 and the feedback switch 315, thepositive input terminals of the comparators 302, 304, 306, 308, 310, and312 receive either the analog input signal or the residual voltagefeedback signal. That is, a selected one of the analog input signal andthe residual voltage feedback signal is input to the positive inputterminals of the first through sixth comparators 302, 304, 306, 308,310, and 312 through the use of the switches 305 and 315. Preferably,the residual voltage feedback signal VR is provided to the comparators302, 304, 306, 308, 310, and 312 via a direct feedback signal path asshown in FIG. 2 (i.e., no intervening circuitry, such as a sample andhold circuit).

Each of the comparators 302, 304, 306, 308, 310, 312 also has a negativeinput terminal that receives a first, second, third, fourth, fifth, andsixth predetermined voltage signals, respectively (e.g., VREF1, VREF2,VREF3, VREF4, VREF5, and VREF6). Each of the first, second, third,fourth, fifth, and sixth comparators 302, 304, 306, 308, 310, and 312compare the signals applied to their respective input terminals togenerate a comparator output signal.

According to an example embodiment, the RSD stage 300 is configurablesuch that, during an A/D conversion process for an analog input signalthat occurs over a number of sequential clock phases, the values of thepredetermined voltage signals (VREF1, VREF2, VREF3, VREF4, VREF5, andVREF6) may be selectively changed for each one of the clock phases. Forexample, during a first clock phase of the analog to digital conversion,each of the first, second, third, fourth, fifth, and sixth predeterminedvoltage signals (VREF1, VREF2, VREF3, VREF4, VREF5, and VREF6) may eachbe set to a unique value. During second and subsequent clock phases ofthe analog to digital conversion, some or all of the first, second,third, fourth, fifth, and sixth predetermined voltage signals (VREF1,VREF2, VREF3, VREF4, VREF5, and VREF6) may be changed to have adifferent value then in a previous clock phase.

According to the example embodiment, during clock phases of the A/Dconversion after the first clock phase, the RSD stage 300 uses outputsfrom less than all of the comparators 302, 304, 306, 308, 310, and 312.In other words, for clock phases after the first clock phase, theresolution that is achieved from the single-bit/multi-bit RSD stage 300is reduced relative to the resolution of the first clock phase. Theseaspects of the example embodiment are described in greater detail below.

The outputs of the first, second, third, fourth, fifth, and sixthcomparators 302, 304, 306, 308, 310, and 312 are connected to a logiccircuit 320. During clock phases of an A/D conversion process, the logiccircuit 320 is capable of generating a digital output signal that isrepresentative of the selected one of either the analog input signal orthe residual voltage feedback signal. In an example embodiment, thelogic circuit 320 generates three raw digital bits (D0, D1, D2) as thedigital output signal during a clock phase of an A/D conversion processbased upon the output from all the comparators 302, 304, 306, 308, 310,312. In the example embodiment, the logic circuit 320 generates two rawdigital bits (D0, D1) as the digital output signal during another clockphase of the A/D conversion process based upon outputs from less thanall of the comparators 302, 304, 306, 308, 310, 312. In a preferredembodiment, the three digital bits (D0, D1, D2) are generated during thefirst clock phase of the A/D conversion process. The digital bitsgenerated during any clock phase of the A/D conversion are aligned andsynchronized in the digital section 220, and then combined with thedigital bit or bits from other clock phases of the A/D conversion toform a formatted binary output code.

During clock phases of the A/D conversion, the logic circuit 320 is alsocapable of generating a high switch control signal 333, a mid switchcontrol signal 353, and a low switch control signal 343 based upon atleast two of the output signals from the comparators 302, 304, 306, 308,310, and 312.

The single-bit/multi-bit RSD stage 300 additionally includes aprogrammable gain/summing element 325. The programmable gain/summingelement 325 receives as inputs the high switch control signal 333, themid switch control signal 353, the low switch control signal 343, theselected one of the analog input signal and the residual voltagefeedback signal from node 307, a first reference voltage VREFP, a secondreference voltage VREFM, and a ground voltage. The programmablegain/summing element 325 generates the residual voltage feedback signalVR. Although the actual transfer function associated with theprogrammable gain/summing element 325 will be dependent upon theparticular design, generally speaking, the residual voltage feedbacksignal VR may be thought of as a sum of two products. Depending on theparticular clock phase of the A/D conversion, the first product iseither the analog input signal or a previously generated value of theresidual voltage feedback signal, multiplied by a first gain factor. Thesecond product is a selected one of the reference voltages (VREFP,VREFM, or zero), multiplied by a second gain factor.

The feedback switch 315 is provided for selecting the residual voltagefeedback signal as an input to the programmable gain/summing element 325and the comparators 302, 304, 306, 308, 310, and 312. The feedbackswitch 315 is disposed between the output of the programmablegain/summing element 325 and the node 307. When the feedback switch 315is closed, the first switch 305 is open so that the residual voltagefeedback signal is input to the programmable gain/summing element 325and the comparators 302, 304, 306, 308, 310, and 312. When the firstswitch 305 is closed, the feedback switch 315 is open so that the analoginput signal is input to the programmable gain/summing element 325 andthe comparators 302, 304, 306, 308, 310, and 312. As discussed above,the first switch 305 is closed in a first clock cycle during theconversion of the analog input signal and the first switch 305 is openfor subsequent cycles of converting the analog input signal.

FIGS. 3 and 4 are schematic circuit diagrams that illustrate in furtherdetail the multi-bit/single-bit RSD stage 300 of FIG. 3 in accordancewith an example embodiment. FIG. 3 illustrates a sub-ADC 400 inaccordance with an example embodiment, while FIG. 4 illustrates in moredetail gain/summing element 325 that may also be called a MultiplyingDigital-to-Analog Converter (MDAC) 325. The multi-bit/single-bit RSDstage 300 illustrated in FIG. 2 may be implemented with the sub-ADC 400of FIG. 3 and the MDAC 325 of FIG. 4.

Referring to FIG. 3, the sub-ADC 400 includes the input terminal 205that receives an analog input signal (VIN). The first switch 305 isdisposed between the input terminal 205 and a first node 405. The firstswitch 305 is operable to selectively apply the analog input signal tothe first node 405 when the first switch is closed. The feedback switch315 is disposed between the first node 405 and a second node 410. Thefeedback switch 315 is operable to selectively apply the residualvoltage feedback signal (VR) to the first node 405 when the feedbackswitch is closed. As was discussed above, when the first switch 305 isclosed, the feedback switch 315 is open and when the first switch 305 isopen, the feedback switch 315 is closed. The first switch 305 is closedduring a first clock phase of an A/D conversion process and the feedbackswitch 315 is closed during subsequent clock phases of the A/Dconversion process.

The sub-ADC 400 further includes first, second, third, fourth, fifth,and sixth comparators 302, 304, 306, 308, 310, and 312, respectively.The operation of the comparators 302, 304, 306, 308, 310, 312 is thesame as what was described above for FIG. 3. The sub-ADC 400 furtherincludes the logic circuit 320. The logic circuit 320 is connected tothe first through sixth comparators 302, 304, 306, 308, 310, 312 andreceives the output signals from the first through sixth comparators.

During the first clock phase of an A/D conversion process, the logiccircuit 320 generates three raw digital bits (D0, D1, D2) based upon theoutput signals from each of the first through sixth comparators 302,304, 306, 308, 310, 312. According to the example embodiment, the logiccircuit 320 generates two raw digital bits (D0, D1) during one or moresubsequent clock phases of the A/D conversion process, based upon theoutput signals from less than all of the comparators 302, 304, 306, 308,310, 312. The logic circuit also generates control signals (h, l, m),which are used to control some switches of the MDAC 325. This will beexplained in further detail below. The control signals h, l, mcorrespond to the high, low, and mid switch control signals 333, 343,and 353 of FIG. 3, respectively.

The programmable gain/summing element 325 of FIG. 2 is implemented usingthe MDAC 325 of FIG. 4. Referring to FIG. 4, the MDAC 325 includes anoperational amplifier (op-amp) 555, op-amp 606, capacitors 521, 523,525, 527, and 581, and switches 502, 504, 506, 508, 512, 514, 522, 524,532, 534, 536, 542, 544, 546, 552, 554, 556, 562, 564, 572, 574, 582,584, 586, 588, 590, 592, 594, 601, 602, 603, 604, and 605, all of whichare arranged in the manner shown in FIG. 4. For completeness, it shouldbe noted at this point that other MDAC designs that implement theprogrammable gain/summing element 325 of FIG. 2 may be conceived bythose of skill in the art that are structurally dissimilar to MDAC 325but nevertheless accomplish the same function. The switches shown inFIG. 4 may be implemented using transistors.

The circuit connections illustrated in FIG. 4 are as follows. The analoginput signal at node 501 is selectively connected to the nodes 511, 513,515, 517 by way of the switches 502, 504, 506, and 508, respectively.The residual voltage feedback signal at node 503 is selectivelyconnected to the nodes 511, 513, 515, 517 by the switches 512, 514, 522,and 524, respectively. The nodes 513, 515, and 517 are selectivelyconnected to the predetermined high reference voltage source (VREFP) bythe switches 532, 534, and 536, respectively. The nodes 513, 515, and517 are selectively connected to the predetermined low reference voltagesource (VREFM) by the switches 542, 544, and 546, respectively. Thenodes 513, 515, and 517 are selectively connected to the predeterminedzero voltage source by the switches 552, 554, and 556, respectively. Thenode 511 is selectively connected to the node 503 by the switch 562.Capacitor 521 is coupled between the nodes 517 and 533. Capacitor 523 iscoupled between the nodes 515 and 533. Capacitor 525 is coupled betweenthe nodes 513 and 531. Capacitor 527 is coupled between the nodes 511and 531. Node 531 is selectively connected to the predetermined zerovoltage by switch 590. Node 533 is selectively connected to thepredetermined zero voltage by switch 592. Node 531 is selectivelyconnected to node 533 by switch 582. Node 531 is selectively connectedto the negative input terminal of the op-amp 555 or op-amp 606 byswitches 564, 601, and 603, while node 533 is selectively connected tothe negative input terminal of op-amp 555 or op-amp 606 and by theswitches 574, switches 601, 603. Node 515 is selectively connected tothe node 503 through switch 572. The negative input terminal of theop-amp 555 or op-amp 606 is selectively connected to the node 583 by theswitches 586, 601, and 603. The negative input terminal of the op-amp555 or 606 is selectively connected to the node 503 by the switches 594,601, and 603. The positive input terminals of the op-amps 555 and 606are tied to the predetermined zero voltage. The capacitor 581 isselectively coupled between the node 503 and the node 583 by the switch588. Node 503 is selectively coupled to the outputs of op-amps 555 or606 by switches 602 and 605. The output of op-amp 606 is selectivelycoupled to the positive input terminal of op-amp 606 by switch 604. Node583 is selectively coupled to the predetermined zero voltage by theswitch 584.

According to an example embodiment, during a clock phase of an A/Dconversion process the MDAC 325 is operable to produce a first gainfactor of four (4) for the analog input signal and a selected secondgain factor of zero, one, two, or three (0, 1, 2, or 3) for a selectedone of the reference voltages (VREFP, VREFM, or zero). According to theexample embodiment, during subsequent clock phases of the A/D conversionthe MDAC 325 is operable to produce a first gain factor of two (2) forthe residual voltage feedback signal and a selected second gain factorof zero or one (0 or 1) for a selected one of the reference voltages(VREFP, VREFM, or zero).

Switches 305 and 315 of FIG. 4, as well as the switches included in theMDAC 325 of FIG. 4, control the operation of the multi-bit/single-bitRSD stage 300. As will become apparent in the following paragraphs, someof the switches in the MDAC 325 are controlled using control signalsthat are derived from a common clock signal, while other switches arecontrolled by the high, low, and mid-switch control signals (h, l, m)that are generated by the logic circuit 320 of the sub-ADC 400. Thedetails associated with deriving one or several control signals from aclock signal is not explained in further detail here, as they are notcritical for an understanding of the example embodiments.

The switches that are included in the MDAC 325 of FIG. 4 are all listedin the left column of Table I, which appears below this paragraph.Control signals are listed in the right column of Table I. For eachswitch or group of switches appearing in the rows of the left column,the corresponding row in the right column contains the control signal orcontrol signals that determine the state of the switch or group ofswitches. The (OR) notation for switches 590 and 592 refers to thelogical OR function.

TABLE I Switch(es) Control Signal 502, 504, 506, 508 SWVIN 512, 514SWFB1 522, 524 SWFB2 532, 534, 536 h (from logic circuit 320) 542, 544,546 l (from logic circuit 320) 552, 554, 556 m (from logic circuit 320)562, 564 MFB1 572, 574 MFB2 582, 584 H_(O) 586 H_(E) 588 H_(SH) 590SWVIN (OR) SWFB1 592 SWVIN (OR) SWFB2 594 reset 601, 602, 604 SWVIN (OR)H_(O) 603, 605 NOT (SWVIN (OR) H_(O))

FIG. 5 is an example timing diagram illustrating control signals thatmay be used to perform an example 10-bit A/D conversion using thesub-ADC 400 of FIG. 3 and the MDAC 325 of FIG. 4. FIG. 6 illustrates aclock signal, as well as control signals that were named in Table I thatare derived from the clock signal. In FIG. 5, one clock cycle is definedas the time between adjacent rising edges of the clock signal. A clockphase is each “up” or “down” period of the clock signal. Thus, FIG. 6illustrates control signals from Table II for ten sequential clockphases or five clock cycles.

Table II, which appears below this paragraph, illustrates the state ofall the switches that are controlled by the control signals of FIG. 5during each of the ten clock phases illustrated in FIG. 6. In Table II,an “X” indicates that the associated switch or switches are closed,while the absence of an entry indicates that the associated switch orswitches are open. Table II was derived using Table I and FIG. 5. Forexample, according to Table 1, the state of switch 592 is controlledbased upon the logical expression SWVIN OR SWFB2. These signals may havecomplements and the complements have “NOT” preceding the signal name.FIG. 5 illustrates that the control signal SWVIN or the control signalSWFB2 is at a logic “high” state during the first, fourth, sixth, andeighth clock phases. Consequently, Table II illustrates that switch 592is closed during the first, fourth, sixth, and eighth clock phases. Thestate of the other switches may be derived in the same manner. Table IItherefore provides a convenient way to summarize the state of switchesthat are illustrated in FIG. 4 during the ten clock phases illustratedin FIG. 5.

TABLE II Clock Phase Switch(es) 1 2 3 4 5 6 7 8 9 10 502, 504, X 506,508 512, 514 X X X 522, 524 X X X 562, 564 X X X 572, 574 X X X 582, 584X 586 X 588 X X 590 X X X X 592 X X X X 601, 602, X X 604 603, 604 X X XX X X X X

FIGS. 6-14 are simplified circuit diagrams illustrating theconfiguration of the sub-ADC 400 of FIG. 3 and the MDAC 325 of FIG. 4for the first nine clock phases of the example 10-bit A/D conversionusing the control signals illustrated in FIG. 5. The simplified circuitdiagrams of FIGS. 7-15 may be obtained using the status of the switchesduring each phase of the 10-bit A/D conversion as summarized in TableII. Thus, FIG. 6 corresponds to the first clock phase, FIG. 7corresponds to the second clock phase, FIG. 8 corresponds to the thirdclock phase, etc., up to FIG. 14, which corresponds to the ninth clockphase. A diagram corresponding to the tenth clock phase omitted because,as can be seen in Table II, all switches are open resulting in anuninteresting configuration for MDAC 325. In order to increase clarity,none of the switches illustrated in FIG. 4 are shown in the simplifiedcircuit diagrams of FIGS. 6-14, and any capacitor 521, 523, 525, 527,581 that is grounded on both sides during a particular clock phase isnot illustrated in the FIGURE corresponding to that clock phase. In theparagraphs that follow, the example 10-bit A/D conversion process asperformed by the sub-ADC 400 of FIG. 3 and the MDAC 325 of FIG. 4 willbe explained in greater detail.

FIG. 6 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC 400 and the MDAC 325 during the first clock phase of theexample 10-bit A/D conversion process. During the first clock phase,switches 601, 602, and 604 are closed and switches 603 and 605 are openwhich results in op-amp 555 being active and op-amp 606 being decoupled.When an op-amp is decoupled it is also powered down to avoid usingcurrent. This is conveniently done by disabling the bias. During thefirst clock phase, the analog input is sampled by the MDAC 325 and thesub-ADC 400, which uses outputs from all of the comparators 302, 304,306, 308, 310, and 312 to generate three bits of raw digital data whichare sent to the digital section 220 of FIG. 1. During the first clockphase, the reset signal is asserted to close the switch 594 of FIG. 4,which results in the resetting of op-amp 555. According to otherembodiments, the op-amp 555 may be reset during the first clock phasefor any n-bit A/D conversion process.

FIG. 7 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC 400 and the MDAC 325 during the second clock phase of theexample 10-bit A/D conversion process. During the second clock phase,switches 601, 602, and 604 are closed and switches 603 and 605 are openwhich results in op-amp 555 being active and op-amp 606 being decoupled.During the second clock phase, the op-amp 555 generates the residualvoltage feedback signal VR, based upon the full sub-ADC 400 results fromthe previous first clock phase. As was explained above, VR, is generatedusing four as the first gain factor for the analog input signal andusing two as the second gain factor for a selected one of thepredetermined reference voltage sources (VREFP, VREFM, zero). Thecapacitors 521, 523, and 525 are tied to either the predetermined highreference voltage source (VREFP), the predetermined low referencevoltage source (VREFM), or the predetermined zero reference voltagebased upon the high, low, and the mid switch control signals (h, l, m).As was explained above, the high, low, and mid switch control signals h,l, m determine the state of the switches 532, 534, 536, 542, 544, 546,552, 554, 556 (FIG. 4) of the MDAC 325. During the second clock phase,the residual voltage feedback signal VR, is sampled on to the capacitor581. Note that during the second clock phase, the sub-ADC 400 does notgenerate a digital output signal of one or more raw digital bits.According to other embodiments, the sub-ADC 400 does not generate adigital output signal during the second clock phase for any n-bit A/Dconversion process.

FIG. 8 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC 400 and the MDAC 325 during the third clock phase of theexample 10-bit A/D conversion process. During the third clock phase,switches 601, 602, and 604 are open and switches 603 and 605 are closedwhich results in op-amp 606 being active and op-amp 555 being decoupled.During the third clock phase, the residual voltage feedback signal VR,generated during the previous second clock phase is held and sampled onto the capacitors 525 and 527 as well as sampled by the comparators 302and 304 of the sub-ADC 400. During the third clock phase, the firstpredetermined voltage signal (VREF1) may be a predetermined high voltage(VH) that is applied to the negative input terminal of the firstcomparator 302. During the third clock phase, the second predeterminedvoltage signal (VREF2) may be a predetermined low voltage (VL) that isapplied to the negative input terminal of the second comparator 304. Theactual voltage values for VH and VL are a function of process technologysince that may limit power supply voltages. However, in one exampleembodiment, VH is about 1.5 Volts (V) and more preferably about 1.475 V,while VL is about 1.2 V and more preferably about 1.225 V. Based uponthe output signals from the comparators 302, 304, the logic circuit 320of the sub-ADC 400 may generate new values for the high, low, and midswitch control signals (h, l, m). The sub-ADC 400 also generates two rawdigital bits at the end of the third clock phase, which are sent to thedigital section 220 of FIG. 1. It will be appreciated by those ofordinary skill that the resolution achieved by the single-bit/multi-bitstage 300 during this clock phase is only 1.5 bits, since only twocomparators 302, 304 of the sub-ADC 400 are used. At the third clockphase the need for resolution is reduced by a factor of four. Op-amp 606is used and op-amp 555 is decoupled to save power by taking advantage ofthe reduced need for resolution. Op-amp 606 has a current savings ofabout a factor of four compared to op-amp 555. The lower current has theeffect of reducing resolution by about the same factor. Since the needfor resolution is reduced, reducing the power is available. As analternative of switching op-amps, during the first two clock phases twoamplifiers could be coupled in parallel in which they have about a threeto one ratio in resolution so that there is a net factor of fourresolution. At the third clock phase, the factor of three op-amp isdecoupled. The effect is the same in that there is a factor of fourreduction in resolution and a corresponding drop in current. Theexpectation is that the op-amp change should reduce the power consumedin the op-amp operation by at least a factor of about four. Anotherpossibility for achieving the at least about 4 times reduction incurrent is to change the bias current of the op-amp 555 at the thirdclock phase and not even have op-amp 606.

FIG. 9 is a simplified circuit diagram illustrating the configuration ofthe sub-ADC 400 and the MDAC 325 during the fourth clock phase of theexample 10-bit A/D conversion process. During the fourth clock phase,the op-amp 555 generates a new residual voltage feedback signal VR₂ fromthe previous residual voltage feedback signal VR, based on the controlsignals (h, l, m) from the previous third clock phase. The residualvoltage feedback signal VR₂ is held and sampled on to the capacitors 521and 523 as well as sampled by the comparators 306, 308 of the sub-ADC400. During the fourth clock phase, the third predetermined voltagesignal (VREF3) may be the predetermined high voltage (VH) that isapplied to the negative input terminal of the third comparator 306.During the fourth clock phase, the fourth predetermined voltage signal(VREF4) may be the predetermined low voltage (VL) that is applied to thenegative input terminal of the fourth comparator 308. Based upon theoutput signals from the comparators 306, 308, the sub-ADC 400 maygenerate new values for the high, low, and mid-switch control signals(h, l, and m). The sub-ADC 400 also generates two raw digital bits atthe end of the fourth clock phase, which are sent to the digital section220 of FIG. 1.

FIG. 10 is a simplified circuit diagram illustrating the configurationof the sub-ADC 400 and the MDAC 325 during the fifth clock phase of theexample 10-bit A/D conversion process. During the fifth clock phase, theop-amp 555 generates a new residual voltage feedback signal VR₃ from theprevious residual voltage feedback signal VR₂ based on the controlsignals (h, l, m) from the previous fourth clock phase. The residualvoltage feedback signal VR₃ is held and sampled on to the capacitors 525and 527 as well as sampled by the comparators 310, 312 of the sub-ADC400. During the fifth clock phase, the fifth predetermined voltagesignal (VREF5) may be the predetermined high voltage (VH) that isapplied to the negative input terminal of the fifth comparator 310.During the fifth clock phase, the sixth predetermined voltage signal(VREF6) may be the predetermined low voltage (VL) that is applied to thenegative input terminal of the sixth comparator 312. Based upon theoutput signals from the comparators 310, 312, the sub-ADC 400 maygenerate new values for the high, low, and mid-switch control signals h,l, and m. The sub-ADC 400 also generates two raw digital bits at the endof the fifth clock phase, which are sent to the digital section 220 ofFIG. 1.

FIG. 11 is a simplified circuit diagram illustrating the configurationof the sub-ADC 400 and the MDAC 325 during the sixth clock phase of theexample 10-bit A/D conversion process. During the sixth clock phase, theop-amp 555 generates a new residual voltage feedback signal VR₄ from theprevious residual voltage feedback signal VR₃ based on the controlsignals (h, l, m) from the previous fifth clock phase. The residualvoltage feedback signal VR₄ is held and sampled on to the capacitors 521and 523 as well as sampled by the comparators 302, 304 of the sub-ADC400. During the sixth clock phase, the first predetermined voltagesignal (VREF1) may be the predetermined high voltage (VH) that isapplied to the negative input terminal of the first comparator 302.During the sixth clock phase, the second predetermined voltage signal(VREF2) may be the predetermined low voltage (VL) that is applied to thenegative input terminal of the sixth comparator 304. Based upon theoutput signals from the comparators 302, 304 the sub-ADC 400 maygenerate new values for the high, low, and mid-switch control signals(h, l, and m). The sub-ADC 400 also generates two raw digital bits atthe end of the sixth clock phase, which are sent to the digital section220 of FIG. 1.

FIG. 12 is a simplified circuit diagram illustrating the configurationof the sub-ADC 400 and the MDAC 325 during the seventh clock phase ofthe example 10-bit A/D conversion process. During the seventh clockphase, the op-amp 555 generates a new residual voltage feedback signalVR₅ from the previous residual voltage feedback signal VR₄ based on thecontrol signals (h, l, m) from the previous sixth clock phase. Theresidual voltage feedback signal VR₅ is held and sampled on to thecapacitors 525 and 527 as well as sampled by the comparators 306, 308 ofthe sub-ADC 400. During the seventh clock phase, the third predeterminedvoltage signal (VREF3) may be the predetermined high voltage (VH) thatis applied to the negative input terminal of the third comparator 306.During the seventh clock phase, the fourth predetermined voltage signal(VREF4) may be the predetermined low voltage (VL) that is applied to thenegative input terminal of the fourth comparator 308. Based upon theoutput signals from the comparators 306, 308 the sub-ADC 400 maygenerate new values for the high, low, and mid-switch control signals(h, l, and m). The sub-ADC 400 also generates two raw digital bits atthe end of the seventh clock phase, which are sent to the digitalsection 220 of FIG. 1.

FIG. 13 is a simplified circuit diagram illustrating the configurationof the sub-ADC 400 and the MDAC 325 during the eighth clock phase of theexample 10-bit A/D conversion process. During the eighth clock phase,the op-amp 555 generates a new residual voltage feedback signal VR₆ fromthe previous residual voltage feedback signal VR₅ based on the controlsignals (h, l, m) from the previous seventh clock phase. The residualvoltage feedback signal VR₆ is held and sampled on to the capacitors 521and 523 as well as sampled by the comparators 310, 312 of the sub-ADC400. During the eighth clock phase, the fifth predetermined voltagesignal (VREF5) may be the predetermined high voltage (VH) that isapplied to the negative input terminal of the fifth comparator 310.During the eighth clock phase, the sixth predetermined voltage signal(VREF6) may be the predetermined low voltage (VL) that is applied to thenegative input terminal of the sixth comparator 312. Based upon theoutput signals from the comparators 310, 312 the sub-ADC 400 maygenerate new values for the high, low, and mid-switch control signals(h, l, and m). The sub-ADC 400 also generates two raw digital bits atthe end of the eighth clock phase, which are sent to the digital section220 of FIG. 1.

FIG. 14 is a simplified circuit diagram illustrating the configurationof the sub-ADC 400 and the MDAC 325 during the ninth clock phase of theexample 10-bit A/D conversion process. During the ninth clock phase, theop-amp 555 generates a new residual voltage feedback signal VR₇ from theprevious residual voltage feedback signal VR₆ based on the controlsignals (h, l, m) from the previous eighth clock phase. The residualvoltage feedback signal VR₇ is held and sampled on to the capacitors 527and 525 as well as sampled by the comparators 302, 304, 306, 308 of thesub-ADC 400. During the ninth clock phase, the first and thirdpredetermined voltage signals (VREF1, VREF3) may be the predeterminedhigh voltage (VH) that is applied to the negative input terminals of thefirst and third comparators 302, 306. During the ninth clock phase, thesecond and fourth predetermined voltage signals (VREF2, VREF4) may bethe predetermined low voltage (VL) that is applied to the negative inputterminals of the second and fourth comparators 304, 308. Based upon theoutput signals from the comparators 302, 304, 306, 308 the sub-ADC 400may generate new values for the high, low, and mid-switch controlsignals (h, l, and m). The sub-ADC 400 also generates two raw digitalbits at the end of the ninth clock phase, which are sent to the digitalsection 220 of FIG. 1.

As explained above, the raw digital bits obtained from the sub-ADC 400in the example 10-bit A/D conversion were sent to the digital section220 of FIG. 1 during the first clock phase and during the third throughninth clock phases. In particular, the raw digital bits are sent to thealignment and synchronization block 230 of FIG. 1, where they arealigned and synchronized. During the tenth clock phase of the example10-bit A/D conversion process, a digital correction is performed in thecorrection block 240 to produce a 10-bit binary word at the end of thetenth clock phase, completing the example 10-bit A/D conversion. Theprocess may then be repeated in the manner described above to produce asecond 10-bit binary word. An observant reader will recognize that thenumber of raw digital bits obtained from the sub-ADC 400 in the example10-bit A/D conversion described above was actually greater than 10 bits.This discrepancy is accounted for because in each one of the clockphases that results in raw digital bits being output from the sub-ADC400, one of the raw bits is redundant and is discarded during furtherprocessing in the digital section 220. Thus, three raw digital bits fromthe sub-ADC during one clock phase produced two bits for the 10-bit A/Dconversion, and two raw digital bits from the sub-ADC during one clockphase produced one bit of the 10-bit A/D conversion.

According to the example 10-bit A/D conversion described above, fiveclock cycles are needed to produce a 10-bit binary word. Thus,generalizing to any n-bit A/D conversion where n is even, an n-bitbinary word may be produced in n/2 clock cycles. In alternativeembodiments, the sub-ADC 400 and the MDAC 325 could be configured toproduce two raw digital bits during, for example, the ninth clock phasethat was described above as producing three raw digital bits for theexample 10-bit A/D conversion. Thus, generalizing to any n-bit A/Dconversion where n is odd, an n-bit binary word may be produced in(n+1)/2 clock cycles. The number of clock cycles used in the example10-bit conversion described above is not significantly different fromthe number of clock cycles required by the single multi-bit A/Dconverter described in U.S. Pat. No. 6,535,157, which may produce twodigital bits during one clock phase of every clock cycle. However, thoseskilled in the art will appreciate that because the sub-ADC 400 and theMDAC 325 of the example embodiment can be continually reconfigured toproduce two raw digital bits during every clock phase for a clock cycleafter an initial clock cycle, the example embodiment can achieve thesame performance with reduced thermal noise, area, and power.

For example, in the 10-bit conversion described above, the first clockphase of the first clock cycle and the ninth clock phase of the fifthclock cycle were used to produce three raw digital bits from the sub-ADC400. The sub-ADC 400 was not used during the second clock phase of thefirst clock cycle. In the second through fourth clock cycles, however,by efficiently reconfiguring circuitry in the MDAC 325 during each clockphase to perform a different function, the sub-ADC 400 was used duringeach clock phase to produce two raw digital bits in each clock phase.Thus, according to example embodiments a single RSD A/D conversion stagecan be initially configured to output at least three raw bits during aninitial conversion clock cycle, then be subsequently reconfigured tooutput two raw bits during every clock phase of subsequent conversionclock cycles in order to determine the remaining bits of the A/Dconversion with reduced capacitance, reduced area, and reduced powerrequirements.

Based on the above, it should be apparent that example embodimentsinclude a single RSD stage that can be selectively reconfigured to havedifferent bit resolutions during different clock phases or clock cyclesof an A/D conversion process. In the particular example described above,the initial resolution was 2.5 bits, and the subsequent resolution was1.5 bits.

The example embodiment described above can achieve the same sample rateand resolution as the architecture described in U.S. Pat. No. 6,535,157,but the reconfiguration from a 2.5 bit resolution stage in the firstconversion clock cycle to a 1.5 bit resolution stage in subsequent clockcycles as described above enables it to do so with approximately a 40%reduction in total capacitance due to reduced thermal noise andapproximately a 25% reduction in area and power.

FIG. 15 is a flowchart illustrating a few example processes included ina method according to an example embodiment. Referring to FIG. 15, amethod 1600 according to an example embodiment starts with process 1610.Process 1610 includes generating, with a single RSD stage, at leastthree bits of raw digital data during a first clock phase of an A/Dconversion of an analog signal. Process 1620 occurs after process 1610,and includes generating, with the same single RSD stage, two bits ofdigital data during a second clock phase of the A/D conversion.

While the order of processes 1610 and 1620 as illustrated in FIG. 15 ispreferred, alternative embodiments may instead reverse the order,placing process 1620 prior to process 1610. In alternative embodiments,there may also be at least one intervening clock phase between the firstclock phase and the second clock phase of the A/D conversion of theanalog signal. In other words, the second clock phase of process 1620does not necessarily occur immediately after the first clock phase ofprocess 1610. It should also be appreciated that the first clock phaseof process 1610 need not be the sequentially first clock phase in anyparticular A/D conversion process, although this is preferred.

Thus, it is seen that a power savings is achieved by reducing current ofan amplifier in MDAC at a time when resolution is reduced. For a givenconversion of a sample to a digital signal, the amplifier has an initialresolution at a relatively higher current requirement and then switchesto a lower resolution at a lower current requirement at a time whenthere is a drop in the resolution requirement. This is a one time changeduring a given conversion of a sample to a digital signal.

It should be apparent by know that a converter adapted to convert ananalog input signal into a digital output signal has been disclosed. Theconverter includes an analog input terminal for receiving the analoginput signal. The converter includes a Redundant Signed Digit (RSD)stage coupled to the analog input terminal. The RSD stage is configuredto receive the analog input signal at the analog input terminal, producea first number of bits at a digital output from the analog input signalduring a first half of a first clock cycle, provide a residual feedbacksignal of the analog input signal at the analog input terminal during asecond half of the first clock cycle and produce a second number of bitsat the digital output from the residual feedback signal during a firsthalf of a second clock cycle, the second number of bits less than thefirst number of bits. The converter includes a digital section coupledto the digital output, the digital section configured to perform adigital alignment and correction on the first number of bits and thesecond number of bits to generate the digital output signal. Theconverter may further comprise a Multiplying Digital to Analog Converter(MDAC) that is operable to produce the residual feedback signal and asubsidiary Analog to Digital Converter (sub-ADC) that is operable toproduce the first number of bits based upon the analog input signal andthat is operable to produce the second number of bits based upon theresidual feedback signal. The converter may further comprise a pluralityof comparators coupled to the analog input terminal and configured tocompare the residual feedback signal to a plurality of predeterminedvoltages and a logic circuit coupled to the plurality of comparators andconfigured to generate the first number of bits based at least uponoutputs from a first set of the comparators, the logic circuit furtherconfigured to generate the second number of bits based at least uponoutputs from a second set of comparators, the second set of comparatorsa subset of the first set of comparators. The converter may furthercomprise an operational amplifier (op-amp) operable to generate theresidual feedback signal, a first capacitor that is coupled to a firstnode, a second capacitor that is coupled to the first node, a thirdcapacitor that is coupled to a second node, a fourth capacitor that iscoupled to the second node, a first switch that is coupled between aninput of the op-amp and the first node, and a second switch that iscoupled between the input of the op-amp and the second node. Theconverter may further comprise a third switch coupled between the firstnode and the second node. The converter may have a furthercharacterization by which the RSD stage is further configured to producea third number of bits during a second half of the second clock cycle,the third number equal to the second number. The converter may have afurther characterization by which the first number is three and thesecond number is two.

Also disclosed is a cyclic Redundant Signed Digit (RSD) Analog toDigital (A/D) converter. The converter includes an input terminal forreceiving an analog input signal. The converter includes a first switchconnected between the input terminal and a first node, the first switchoperable to apply the analog input signal to the first node. Theconverter includes a second switch connected between the first node anda second node, the second switch operable to apply a residual voltagefeedback signal to the first node, the first switch operable to beclosed when the second switch is open, the second switch operable to beclosed when the first switch is open. The converter includes anoperational amplifier having an output terminal connected to the secondnode, the operational amplifier operable to generate the residualvoltage feedback signal and apply it to the second node. The converterincludes. The converter includes comparators, each comparator having afirst input coupled to the first node and an output, each of thecomparators operable to compare a selected one of the analog inputsignal and the residual voltage feedback signal to a predeterminedvoltage signal. The converter includes a logic circuit coupled to theoutputs of the comparators, the logic circuit operable to generate afirst digital output signal during a first clock phase of an A/Dconversion and operable to generate a second digital output signalduring a second clock phase of the A/D conversion, the first digitaloutput signal based upon the outputs from a first set of thecomparators, the second digital output signal based upon the outputsfrom a second set of the comparators. The converter may have a furthercharacterization by which the first digital output signal comprisesthree digital bits, the second digital output signal consisting of twodigital bits. The converter may have a further characterization by whichthe second clock phase is subsequent to the first clock phase, andwherein there is at least one intervening clock phase between the firstclock phase and the second clock phase. The converter may have a furthercharacterization by which the second set of the comparators is a subsetof the first set of the comparators. The converter may have a furthercharacterization by which first digital output signal comprises threedigital bits and the second digital output signal consists of twodigital bits. The converter may have a further characterization by whicha clock cycle of the A/D conversion consists of the first clock phaseand the second clock phase.

Also disclosed is a method for converting an analog input signal into aplurality of digital bits during a plurality of clock cycles using asingle Redundant Signed Digit (RSD) stage of an Analog to Digital (A/D)converter. The method includes receiving the analog input signal. Themethod includes producing a first number of the plurality of digitalbits at a first resolution during one of the clock cycles and producinga second number of the plurality of digital bits at a second resolutionduring another one of the clock cycles. The method may have a furthercharacterization by which producing the first number of digital bits andthe second number of digital bits comprises the steps of producing thefirst number of the digital bits from the analog input signal during afirst half of a first clock cycle, producing a first residual voltagefrom the analog input signal during a second half of the first clockcycle, and producing the second number of the digital bits from thefirst residual voltage during a first half of a second clock cycle. Themethod may have a further characterization by which the first resolutionis at least 2.5 bits, and wherein the second resolution is less than thefirst resolution. The method may have a further characterization bywhich the second resolution is 1.5 bits. The method may have a furthercharacterization by which the first half of the second clock cycleoccurs following the second half of the first clock cycle. The methodfurther comprising the step of producing a second residual voltage fromthe first residual voltage during a second half of the second clockcycle. The method further comprising the step of producing a thirdnumber of digital bits at the second resolution from the second residualvoltage during the second half of the second clock cycle.

It shall be apparent to those of ordinary skill, based upon the limitednumber of example embodiments described above, that many otherembodiments that incorporate one or more of the inventive principlesthat were associated with the described example embodiments exist. Inthe following paragraphs, more descriptions of example, non-limitingembodiments are presented.

While at least one example embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist, especially with respect to choices of devicetypes and materials and the sequence of processes. It should further beemphasized that the example embodiments described above are onlyexamples, and are not intended to limit the scope, applicability, orconfiguration in any way. Rather, the detailed description of theexample embodiments provides those skilled in the art with a convenientroad map for implementing the inventive principles contained in theexample embodiments. The inventors regard the subject matter to includeall combinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. It should also beunderstood that various changes can be made in the function andarrangement of elements without departing from the scope as set forth inthe appended claims and the legal equivalents thereof.

1. A converter adapted to convert an analog input signal into a digitaloutput signal, comprising: an analog input terminal for receiving theanalog input signal; a Redundant Signed Digit (RSD) stage coupled to theanalog input terminal, said RSD stage configured to: receive the analoginput signal at the analog input terminal; produce a first number ofbits at a digital output from the analog input signal during a firsthalf of a first clock cycle; provide a residual feedback signal of theanalog input signal at the analog input terminal during a second half ofthe first clock cycle; and produce a second number of bits at thedigital output from the residual feedback signal during a first half ofa second clock cycle, the second number of bits less than the firstnumber of bits; and a digital section coupled to the digital output, thedigital section configured to perform a digital alignment and correctionon the first number of bits and the second number of bits to generatethe digital output signal.
 2. The converter of claim 1, the RSD stagecomprising: a Multiplying Digital to Analog Converter (MDAC) that isoperable to produce the residual feedback signal; and a subsidiaryAnalog to Digital Converter (sub-ADC) that is operable to produce thefirst number of bits based upon the analog input signal and that isoperable to produce the second number of bits based upon the residualfeedback signal.
 3. The converter of claim 2, the sub-ADC comprising: aplurality of comparators coupled to the analog input terminal andconfigured to compare the residual feedback signal to a plurality ofpredetermined voltages; and a logic circuit coupled to the plurality ofcomparators and configured to generate the first number of bits based atleast upon outputs from a first set of the comparators, the logiccircuit further configured to generate the second number of bits basedat least upon outputs from a second set of comparators, the second setof comparators a subset of the first set of comparators.
 4. Theconverter of claim 3, the MDAC comprising: an operational amplifier(op-amp) operable to generate the residual feedback signal; a firstcapacitor that is coupled to a first node; a second capacitor that iscoupled to the first node; a third capacitor that is coupled to a secondnode; a fourth capacitor that is coupled to the second node; a firstswitch that is coupled between an input of the op-amp and the firstnode; and a second switch that is coupled between the input of theop-amp and the second node.
 5. The converter of claim 4 the MDAC furthercomprising a third switch coupled between the first node and the secondnode.
 6. The converter of claim 1, wherein the RSD stage is furtherconfigured to produce a third number of bits during a second half of thesecond clock cycle, the third number equal to the second number.
 7. Theconverter of claim 6, wherein the first number is three and the secondnumber is two.
 8. A cyclic Redundant Signed Digit (RSD) Analog toDigital (A/D) converter comprising: an input terminal for receiving ananalog input signal; a first switch connected between the input terminaland a first node, the first switch operable to apply the analog inputsignal to the first node; a second switch connected between the firstnode and a second node, the second switch operable to apply a residualvoltage feedback signal to the first node, the first switch operable tobe closed when the second switch is open, the second switch operable tobe closed when the first switch is open; an operational amplifier havingan output terminal connected to the second node, the operationalamplifier operable to generate the residual voltage feedback signal andapply it to the second node; comparators, each comparator having a firstinput coupled to the first node and an output, each of the comparatorsoperable to compare a selected one of the analog input signal and theresidual voltage feedback signal to a predetermined voltage signal; anda logic circuit coupled to the outputs of the comparators, the logiccircuit operable to generate a first digital output signal during afirst clock phase of an A/D conversion and operable to generate a seconddigital output signal during a second clock phase of the A/D conversion,the first digital output signal based upon the outputs from a first setof the comparators, the second digital output signal based upon theoutputs from a second set of the comparators.
 9. The cyclic RSD A/Dconverter of claim 8, the first digital output signal comprising threedigital bits, the second digital output signal consisting of two digitalbits.
 10. The cyclic RSD A/D converter of claim 9, wherein the secondclock phase is subsequent to the first clock phase, and wherein there isat least one intervening clock phase between the first clock phase andthe second clock phase.
 11. The cyclic RSD A/D converter of claim 9,wherein the second set of the comparators is a subset of the first setof the comparators.
 12. The cyclic RSD A/D converter of claim 8, thefirst digital output signal comprising three digital bits, the seconddigital output signal consisting of two digital bits.
 13. The cyclic RSDA/D converter of claim 12, wherein a clock cycle of the ND conversionconsists of the first clock phase and the second clock phase.
 14. Amethod for converting an analog input signal into a plurality of digitalbits during a plurality of clock cycles using a single Redundant SignedDigit (RSD) stage of an Analog to Digital (ND) converter, the methodcomprising the steps of: receiving the analog input signal; andproducing a first number of the plurality of digital bits at a firstresolution during one of the clock cycles and producing a second numberof the plurality of digital bits at a second resolution during anotherone of the clock cycles.
 15. The method of claim 14, wherein producingthe first number of digital bits and the second number of digital bitscomprises the steps of: producing the first number of the digital bitsfrom the analog input signal during a first half of a first clock cycle;producing a first residual voltage from the analog input signal during asecond half of the first clock cycle; and producing the second number ofthe digital bits from the first residual voltage during a first half ofa second clock cycle.
 16. The method of claim 14, wherein the firstresolution is at least 2.5 bits, and wherein the second resolution isless than the first resolution.
 17. The method of claim 16, wherein thesecond resolution is 1.5 bits.
 18. The method of claim 15, wherein thefirst half of the second clock cycle occurs following the second half ofthe first clock cycle.
 19. The method of claim 15, further comprisingthe step of producing a second residual voltage from the first residualvoltage during a second half of the second clock cycle.
 20. The methodof claim 19, further comprising the step of producing a third number ofdigital bits at the second resolution from the second residual voltageduring the second half of the second clock cycle.